1. Field of the Invention
The present invention relates to a waveguide, an apparatus including the waveguide, and a method of manufacturing the waveguide. More particularly, the present invention relates to a waveguide for electromagnetic wave in a frequency band from a millimeter wave band to a terahertz wave band (30 GHz to 30 THz) (hereinafter, also referred to as terahertz wave).
2. Description of the Related Art
In the frequency band of terahertz wave, there are absorption peaks of many organic molecules of biological materials, medicines, electronic materials, and the like stemming from the structures and states thereof. Further, terahertz wave easily penetrates materials such as paper, ceramic, a resin, and cloth. In recent years, research and development have been conducted on imaging technology and sensing technology which makes use of such characteristics of terahertz wave. For example, application thereof to a safe fluoroscopic inspection apparatus to replace an X-ray apparatus, to an in-line non-destructive inspection apparatus in a manufacturing process, and the like is expected.
As a current injection type terahertz wave light source, a structure is under study, which uses an electromagnetic wave gain based on intersubband transition of electrons in a semiconductor quantum well structure. Appl. Phys. Lett. 83, 2124 (2003) proposes a terahertz wave band quantum cascade laser (hereinafter, also referred to as QCL) in which double-side metal waveguides (hereinafter, also referred to as DMW) which are known as low loss waveguides are integrated as resonators. This element attains laser oscillation around 3 THz by excellent light confinement and low loss propagation by guiding a terahertz wave that is inductively emitted to a resonator structure in which a metal is placed above and below a gain medium formed of a semiconductor thin film of a thickness of about 10 μm in a surface plasmon mode.
On the other hand, a multiple quantum well structure is known to change the characteristics thereof owing to strain applied thereto. In Sensors and Actuators, A, 143 (2008), 230-236, a report is made that the characteristics of a resonant tunnel diode (hereinafter, also referred to as RTD) change owing to strain applied thereto. In Sensors and Actuators, A, 143 (2008), 230-236, change of about twice in negative differential resistance at a stress of near 100 MPa is observed. Further, a laser device with a waveguide as described in U.S. Pat. No. 7,693,198 is disclosed.
The DMW disclosed in Appl. Phys. Lett. 83, 2124 (2003) has a structure in which two metal layers sandwich a semiconductor layer having a thickness of about 10 μm, and the DMW is manufactured by transferring the semiconductor thin film onto a different substrate using a metal bonding technology or the like. On the other hand, a structure in which thin film materials having different lattice constants and different thermal expansion coefficients are laminated is generally known to be liable to cause a residual stress therein due to the manufacturing process. Therefore, in a conventional structure, the strain or defect due to the manufacturing process or the like may change the characteristics of the semiconductor thin film which is a gain medium to cause deterioration or instability of the oscillation characteristics.